Mehrkernige Kobaltkomplexe. II. Herstellung und Struktur von [(tren) (NH 3 )Co(O 2 )Co(NH 3 ) (tren)](SCN) 4 · 2H 2 O

Polynuclear Cobalt Complexes. II. Preparation and Structure of [(tren) (NH 3 )Co(O 2 )Co(NH 3 ) (tren)](SCN) 4 · 2H 2 O The title compound is obtained on oxygenation of [Co(tren)(H 2 O) 2 ] 2+ in 6 M aqueous ammonia or by ligand exchange starting from [(NH 3 ) 5 Co(O 2 )Co(NH 3 ) 5 ]‐(NO 3 ) 4 . An X‐ray structure determination was made. The substance forms monoclinic crystals, space group P2 1 /c, lattice constants a =10,135, b =8,473, c =19,484 Å, β=108,58°, with two formula units in the cell. The final R is 0,066. The binuclear cation has a center of symmetry, so the CoOOCo unit is planar; the CoOO angle is 111,5°. The tertiary nitrogen atoms of both chelate groups are cis to the O 2 bridge, as found in doubly bridged [(tren)Co(O 2 ,OH)Co(tren)](ClO 4 ) 3 · 3H 2 O. On acidification in solution, the singly bridged cation [(tren) (NH 3 )CoO 2 Co(NH 3 )(tren)] 4+ (a) loses the bound O 2 completely. But unlike the doubly bridged cation b , the rate of dissociation of a is independent of pH (Fig. 5). At higher pH (8–10) bridging a→b (Fig. 2) occurs. Both reactions must have the same rate determining step, the first order rate constants being of the order of 2 · 10 −3 s −1 (25°, 0,35 M KCl).